Electric cable

ABSTRACT

A cable comprising an electrical conductor with insulating and protecting layers surrounding the conductor is described. The cable is characterised in that at least one layer selected from said insulating and protecting layers consists of a crosslinked ethylene-alkyl (meth)acrylate-unsaturated silane terpolymer composition wherein the alkyl (meth)acrylate comonomer comprises more than 5 mole % and the terpolymer composition has a tensile modulus, determined according to ISO 527-2 (1 mm/min) of less than 100 MPa. Preferably, the crosslinked ethylene-alkyl (meth)acrylate polymer is a moisture curable ethylene-alkyl (meth)acrylate-vinyl trialkoxysilane terpolymer, wherein the trialkoxysilan termonomer comprises 0.2-5% by weight of the polymer composition. The polymer composition may include from 0 up to 50% by weight of a plasticiser, up to 60% by weight of a filler, and up to 10% by weight of an additive.

TECHNICAL FIELD

The present invention relates to an electrical cable, and moreparticularly to an electrical rubber cable.

TECHNICAL BACKGROUND

Electrical rubber cables, are flexible electrical cables with aconductor surrounded by an insulating layer and an outer jacketinglayer. Several insulated conductors may be enclosed by one jacket. Suchrubber cables are usually low or medium voltage cables, i.e. they areintended for voltages up to about 10 kV. The insulating and/or jacketinglayers are conventionally made of natural rubber (NR) orethylene-propylene rubber (EPR), e.g. EPDM or EPM.

There are several disadvantages associated with rubber cables. Thus, thefact that they comprise rubbery components means that they are madeaccording to conventional rubber processing procedures. This includeshandling of powdery raw materials, processing of the powdery rawmaterials in special rubber compounding equipment, and curing of thecables by curing procedures involving sulphur or peroxide curing agents.Further, the ageing resistance of rubber cables often leaves somethingto be desired as does the resistance against solvents such as gasolineor oils.

It would therefore be an important progress in the art if a rubber cablecould be obtained that is made of ordinary ethylene polymer material andthat is produced with ordinary polymer processing equipment.

SUMMARY OF THE INVENTION

It is an object of the present invention to ameliorate or eliminate thedrawbacks of the prior art and provide an electrical cable of the rubbercable type where the insulation and/or jacketing is made of acrosslinked rubbery ethylene polymer composition.

It is a further object of the invention to provide an electrical cableof the rubber cable type where the insulating and/or jacketing layer(s)are made of pelleted raw materials that are easy to handle.

It is a still further object of the invention to provide an electricalcable of the rubber cable type where the insulating and/or jacketinglayer(s) are obtained by extruding the materials (s).

It is another object of the invention to provide an electrical cable ofthe rubber cable type which is cross linkable by so-called moisturecuring.

It is still another preferred object of the invention to provide anelectrical cable of the rubber cable type with good ageing and solventresistance.

Thus, according to the present invention there is provided a cablecomprising an electrical conductor with insulating and protecting layerssurrounding the conductor, characterised in that at least one layerselected from said insulating and protecting layers consists of acrosslinked ethylene-alkyl (meth)acrylate-unsaturated silane terpolymercomposition wherein the alkyl (meth)-acrylate comonomer comprises morethan 5 mole % and the terpolymer composition has a tensile modulus,determined according to ISO 527-2 (1 mm/min) of less than 100 MPa.

These and other advantages and characterising features of the presentinvention will appear from the following specification and the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Generally, and in connection with the present invention the expression“alkyl (meth)acrylate” includes alkyl acrylates as well as alkylmethacrylates. The alkyl moiety preferably is an alkyl group having 1-4carbon atoms, such as methyl, ethyl, propyl, and butyl, preferablymethyl or butyl.

Conventional ethylene-alkyl (meth)acrylate polymers generally comprisethe alkyl (meth)acrylate comonomer in a low amount of up to about 10% byweight. The present invention differs from such conventionalethylene-alkyl (meth)acrylate copolymers in that it is not a copolymer,but a terpolymer containing an unsaturated silane compound as atermonomer, and also in that it contains the alkyl (meth)acrylatecomonomer in a high amount of at least 5 mole %, preferably 5-25 mole %.More preferably the alkyl (meth)acrylate comonomer comprises about 9-20mole % of the polymer. The high alkyl (meth)acrylate comonomer contentat the present invention is necessary in order to make the polymercomposition sufficiently soft and flexible.

The terpolymer according to the present invention should have a meltflow rate (MFR₂), determined according to ISO 1133, Condition D, of0.1-40 g/10 min.

In this connection, it is a general requirement for rubber cables tohave a Shore A hardness of less than 85. This requirement thus alsoapplies to the the insulating and jacketing layers of the cableaccording to the present invention. The Shore A hardness is determinedaccording to ISO 868.

Further, the terpolymer composition according to the present inventionshould have a tensile modulus, determined according to ISO 527-2 (1mm/min) of less than 100 MPa, preferably less than 60 MPa, and mostpreferably less than 30 MPa.

As mentioned above, the ethylene-alkyl (meth)acrylate polymercomposition of the insulating or jacketing layer of the inventive cableis crosslinkable.

The crosslinking at the present invention is by way of hydrolysablesilane groups which are incorporated in the ethylene-alkyl(meth)acrylate polymer composition constituting the insulation and/orjacketing layer of the cable according to the invention.

The crosslinking of polymers with hydrolysable silane groups is carriedout by so-called moisture curing. In a first step, the silane groups arehydrolysed under the influence of water or steam, resulting in thesplitting-off of alcohol and the formation of silanol groups. In asecond step, the silanol groups are cross linked by a condensationreaction splitting off water. In both steps, a so-called silanolcondensation catalyst is used as a catalyst.

Silanol condensation catalysts include carboxylates of metals, such astin, zinc, iron, lead and cobalt; organic bases; inorganic acids; andorganic acids. In practice dibutyl tin dilaurate (DBTL) is generallyused as the silanol condensation catalyst.

At the present invention it is preferred, however, to use a specificsilanol condensation catalyst of formula I

ArSO₃H  (I)

or a precursor thereof, Ar being a benzene ring substituted with atleast one hydrocarbyl radical such that the total number of carbon atomsof the hydrocarbyl radical(s) is 8-20, or a naphthalene ring substitutedwith at least one hydrocarbyl radical such that the total number ofcarbon atoms of the hydrocarbyl radical(s) is 4-18, and the catalyst offormula I containing 14-28 carbon atoms in total. This catalyst, asopposed to conventional silanol condensation catalysts such as e.g. DBTLallows crosslinking at ambient temperature such as room temperature.

A silanol condensation catalyst of the above defined type is disclosedin WO 95/17463 for the crosslinking of polymers with hydrolysable silanegroups.

With regard to the silanol condensation catalyst of formula I it ispreferred that the hydrocarbyl radical in formula I is an alkylsubstituent with 10-18 carbon atoms.

The currently most preferred compounds of formula I are dodecyl benzenesulphonic acid and tetrapropyl benzene sulphonic acid.

It is further preferred that the polymer composition includes 0.0001-3%by weight of silanol condensation catalyst.

In the following the crosslinkable hydrolysable silane group containingpolymer used for the insulating and/or jacketing layer compositionaccording to the present invention will described.

The crosslinkable base resin generally is an ethylene copolymer or graftpolymer which contains hydrolysable silane groups and which iscrosslinked under the influence of water and at least one silanolcondensation catalyst. Specifically, the crosslinkable polymer is anethylene-alkyl (meth)acrylate polymer containing crosslinkable silanegroups introduced either by copolymerisation or graft polymerisation.

Preferably, the silane-containing polymer has been obtained bycopolymerisation of ethylene, an alkyl (meth)acrylate comonomer and anunsaturated silane termonomer compound represented by the formula II

RSiR′_(n)Y_(3−n)  (II)

wherein

R is an ethylenically unsaturated hydrocarbyl, hydrocarbyloxy or(meth)acryloxy hydrocarbyl group,

R′ is an aliphatic saturated hydrocarbyl group,

Y which may be same or different, is a hydrolysable organic group, and

n is 0, 1 or 2.

If there is more than one Y group, these do not have to be identical.

Specific examples of the unsaturated silane compound are those wherein Ris vinyl, allyl, isopropenyl, butenyl, cyclohexenyl orgamma-(meth)acryloxy propyl; Y is methoxy, ethoxy, formyloxy, acetoxy,propionyloxy or an alkyl- or arylamino group; and R′, if present, is amethyl, ethyl, propyl, decyl or phenyl group.

A preferred unsaturated silane compound is represented by formula III

CH₂═CHSi(OA)₃  (III)

wherein A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4carbon atoms.

The most preferred compounds are vinyl trimethoxysilane, vinyltriethoxysilane, gamma-(meth)acryloxypropyltrimethoxysilane and vinyltriacetoxysilane or combinations of two or more thereof.

The copolymerisation of the ethylene, the alkyl (meth)acrylate, and theunsaturated silane compound may be carried out under any suitableconditions resulting in polymerisation of the monomers, e.g. asdisclosed in GB 2,088,831.

The silane-containing polymer according to the invention suitablycontains 0.2-5.0% by weight of the silane compound, preferably 0.5-3% byweight.

If using a graft polymer, this may be produced e.g. by the methodsdescribed in U.S. Pat. No. 3,646,155 and U.S. Pat. No. 4,117,195.

The above ethylene-alkyl (meth)acrylate-unsaturated silane terpolymersare produced by radical initiated high pressure polymerisation.Generally, the polymerisation of the monomers is carried out at atemperature of about 100-300° C. and at a pressure of about 100-300 MPain the presence of a radical initiator in a polymerisation reactor.Usually, the polymerisation is carried out continuously, preferably in atubular reactor or in a stirred tank reactor.

Usually, when polymerising ethylene-alkyl (meth)acrylate polymers with ahigh content of (meth)acrylate the polymerisation may be troubled byfouling of the polymerisation reactor which manifests itself as unstableand inhomogenous production. To alleviate this problem and inhibitfouling during the polymerisation an adhesion reducing siliconcontaining compound, e.g. a silane or a silicone compound may be addedas an anti-fouling agent to the polymerisation reactor, as is disclosedin the international patent application PCT/SE98/01949, filed on Oct.28, 1998. At the present invention it is not necessary to add any suchanti-fouling agent, because the terpolymer of the present inventionincludes a silane termonomer which avoids fouling and in practice actsas an anti-fouling agent.

Although ethylene-alkyl (meth)acrylate polymers, such as ethylene-methylacrylate polymers in general and polymers with a high content of alkyl(meth)acrylate comonomer such as the ethylene-alkyl(meth)acrylate-unsaturated silane terpolymer of the present invention inparticular are sufficiently soft and flexible at room temperature, theybecome increasingly stiff and rigid at lower temperatures such assub-zero temperatures. With regard to electrical cables of the rubbercable type these must be soft and flexible not only at room temperature,but also at sub-zero temperatures, such as −20° C. to −30° C. to be ofpractical use.

The present invention has solved this problem by adding a plasticiser tothe ethylene-alkyl (meth)acrylate polymer composition when desired ornecessary. The particular type of plasticiser is not critical to thepresent invention, but it is preferred that the plasticiser is selectedfrom a particular group of plasticisers.

These preferred plasticiser are selected from the group consisting of:alkyl alcoholes; secondary or tertiary amines; esters of carboxylicacids with at least one carboxylic function; amides of mono- ordicarboxylic acids; esters of phosphoric acid; organic oils; and mineraloils.

These plasticisers result in a composition of the desired softness andflexibility both at room temperature and at sub-zero temperatures. Inaddition they are compatible with the polymer and do not migrate fromthe polymer or result in exudation.

Preferably, the plasticiser is selected from the group consisting of:linear or branched C₈-C₁₈ alkyl alcoholes; linear or branched C₄-C₁₈alkyl secondary or tertiary amines; linear or branched C₄-C₁₈ alkylesters of C₆-C₁₀ dicarboxylic acids; C₄-C₁₈ N-substituted amides ofC₁₂-C₁₈ linear monocarboxylic acids or C₆-C₁₀ dicarboxylic acids; alkyl,aryl, alkylaryl, or arylalkyl esters of phosphoric acid where the alkylmoiety is C₆-C₁₈ and the aryl moiety is phenyl; organic oils likesunflower oil, rape seed oil, terpene oil or soybean oil; and mineraloils like paraffinic oil, aromatic oil and, in particular, naphtenicoil.

Among the preferred linear or branched C₈-C₁₈ alkyl alcoholes may bementioned octanols like 2-ethyl-1-hexanol or 1-octanol, 1-decanol and1-dodecanol, etc.

Among the preferred linear or branched C₄-C₁₈ alkyl secondary ortertiary amines compounds like tri-n-butyl amine and di-n-hexyl aminemay be mentioned.

Among the esters of dicarboxylic acids are esters of aliphaticdicarboxylic acids with 6-10 carbon atoms, such as adipic acid, pimelicacid, suberic acid, azelaic acid, and sebacic acid. Preferably, theesters are alkyl esters where the alkyl moiety has 4-18 carbon atoms,such as butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl,dodecyl, tridecyl, etc. Particularly preferred are C₄-C₁₈ alkyl estersof adipic acid.

Among esters of dicarboxylic acids are also esters of aromaticdicarboxylic acids, such as alkyl esters of phthalic acid. As particularexamples of alkyl esters of phthalic acid may be mentioned e.g. dimethylphthalate, diethyl phthalate, dibutyl phthalate (DBP), diisobutylphthalate, dihexyl phthalate, dioctyl phthalate (DOP), diisooctylphthalate, diisononylphthalate, diisodecyl phthalate,diundecylphthalate, ditridecyl phthalate, butyl benzyl phthalate, butyloctyl phthalate, dicapryl phthalate, and dicyclohexyl phthalate.Although these plasticisers may be used at the present invention in viewof their physical properties, they are not preferred, but avoided forenvironmental reasons.

As particular examples of preferred esters of dicarboxylic acids may bementioned diisobutyl adipate, di(n-heptyl, n-nonyl)adipate, dioctyladipate [di(2-ethylhexyl)adipate], dicapryl adipate, diisodecyl adipate,dinonyl adipate, di(tridecyl) adipate, dimetyl sebacate, dibutylsebacate, and di(2-ethylhexyl) sebacate. Of these dioctyl adipate is aparticularly preferred plasticiser.

Among the amides the N-substituted C₄-C₁₈ alkyl amides of C₆-C₁₀dicarboxylic acids compounds corresponding to those of the above C₄-C₁₈alkyl esters of C₆-C₁₀ dicarboxylic acids may be mentioned where theamide moiety is selected from butyl amide, pentyl amide, hexyl amide,heptyl amide, octyl amide, nonyl amide, decyl amide, undecyl amide,dodecyl amide, tridecyl amide, etc.

The esters of phosphoric acid are preferably selected from alkyl,alkoxy, aryl, alkylaryl, or arylalkyl esters of phosphoric acid. Thealkyl or alkoxy moiety of these esters preferably has 6-16 carbon atoms,more preferably 8-14 carbon atoms, and the aryl moiety preferably isphenyl which may be unsubstituted or substituted with C₁-C₄ alkyl ochhydroxyl. Particularly preferred examples of esters of phosphoric acidare alkyldiphenyl phosphates, such as metyldiphenyl phosphate,2-etylhexyl diphenyl phosphate, isodecyl diphenyl phosphate,t-butylphenyl diphenyl phosphate; tributyl-phosphate; tricresylphosphate; triphenyl phosphate; and tributoxiethyl phosphate.

As preferred examples of mineral oil may be mentioned aliphatic,aromatic or , preferably, naphtenic oils.

The plasticisers defined and exemplified above may be used alone or incombination with each other.

The amount of the plasticiser, when present, should be that required toobtain the desired softness and flexibility of the final polymercomposition. The content of the plasticiser is from 0 up to 50% byweight, preferably 5-50% by weight, more preferably 5-30% by weight,most preferably 10-30% by weight, based on the total weight of thecomposition.

The composition of the present invention may further include a filler.Although there is no particular restriction on the choice of filler, itis preferably selected from inorganic fillers. As examples ofparticularly preferred fillers may be mentioned calcium carbonate,kaolin, talc, Mg(OH)₂, and Al(OH)₃. The total amount of filler, whenpresent, is up to 60% by weight, preferably up to 40% by weight, basedon the weight of the total composition.

In order to alleviate or inhibit any problems with scorch duringprocessing (extrusion) of the polymer composition of the presentinvention it may also include a so-called scorch retarding agent. As anexample of such agents may be mentioned silane compounds of the generalformula IV

R¹(SiR² _(n)X_(3−n))_(m)  (IV)

wherein

R¹ is a monomfunctional hydrocarbyl group having 13-30 carbon atoms, ora difunctional hydrocarbyl group having 4-24 carbon atoms,

R² which may be the same or different, is a hydrocarbyl group having1-10 carbon atoms,

X which may be the same or different, is a hydrolysable organic group,

n is 0, 1 or 2, and

m is 1 or 2;

as disclosed in EP 0 449 939.

It should be understood that the composition of the present inventionmay also include conventional additives, such as stabilisers,crosslinking agents, coagents, process aids, etc. The total content ofsuch additives, when present, is up to 10% by weight, preferably up to5% by weight, based on the weight of the total composition.

It should be understood that the sum of the percentages of all thecomponents present in the ethylenealkyl (meth)acrylate polymercomposition of the invention is 100%.

As indicated earlier, an electrical rubber cable having its insulatingand/or jacketing layers made of the above described ethylene-alkyl(meth)acrylate polymer composition is superior to conventional rubbercables in that

a) it is easier to make and causes less pollution than conventionalrubber cables, because the insulating and/or jacketing layers are madeof free flowing pelleted raw materials instead of powdery raw materials;

b) it gives a larger choice of cable production equipment, because it isnot restricted to production in equipment for rubber processing.Equipment for the processing of polyethylene or PVC may be used e.g.;

c) it is easy to crosslink by moisture curing, instead of vulcanisationinvolving peroxide or sulphur as in the curing of rubber. This meansthat there are no problems with peroxide or sulphur smell or restproducts;

d) the moisture curing (silane crosslinking) does not require anyspecial curing equipment;

e) the moisture curing permits a wider process window because of thepossibility to use higher temperatures and less problems with scorch;

f) higher productivity is possible, because the crosslinking operationis no bottleneck;

g) it has a very good ageing resistance, i.e resistance to air, oxygenand ozone and high temperatures;

h) it has a very good resistance to solvents such as gasoline and oils.

Having thus described the present invention above it will now beillustrated by way of a non-limiting example. In the example allpercentages and parts are by weight, unless otherwise stated.

EXAMPLE

An ethylene-methyl acrylate-vinyl trimethoxysilane terpolymer wasprepared by radical initiated high pressure polymerisation in a highpressure tubular reactor. The resulting elastomeric terpolymer contained9 mole % (31% by weight) of methyl acrylate, 1% by weight of vinyltrimethoxysilane and had an MFR_(2.16) of 10 g/10 min, a melttemperature of 71° C., a crystallinity of 7% by weight, a Shore Ahardness of 53, a flexibility in terms of the dynamic shear modulus(determined according to ISO 6721-2A, 23° C.) of 4.2 MPa, and a tensilemodulus of 2.4 MPa.

The terpolymer was mixed with dodecyl benzene sulphonic acid (a silanecondensation catalyst) in a Brabender kneader at 120° C. and 40 rpm for10 min. Then the composition was compression moulded into a 2 mm thickplaque which was immersed in a water bath at 60° C. for 18 h. Thecrosslinked plaque was removed from the water bath and the hot set ofthe crosslinked composition was determined as defined earlier. Thecomposition had a hot set value of 60% which indicates that it wassufficiently crosslinked.

The terpolymer of the Example was used to make a rubber cable byextruding the terpolymer around a metallic conductor and moisture curingthe terpolymer after the extrusion.

What is claimed is:
 1. An electrical rubber cable comprising anelectrical conductor with insulating and jacketing layers surroundingthe conductor, characterised in that at least one said insulating andjacketing layers consists of a crosslinked ethylene-alkyl(meth)acrylate-unsaturated silane terpolymer composition wherein thealkyl (meth)acrylate comonomer comprises more than 5 mole % and theterpolymer composition has a tensile modulus, determined according toISO 527-2 (1 mm/min) of less than 100 MPa.
 2. A cable as claimed inclaim 1, wherein the terpolymer composition has a tensile modulus ofless than 60 MPa.
 3. A cable as claimed in claim 1, wherein theterpolymer composition has a tensile modulus of less than 30 MPa.
 4. Acable as claimed in claim 1, wherein the terpolymer is an ethylene-alkyl(meth)acrylate-vinyl trialkoxysilane terpolymer.
 5. A cable as claimedin claim 4, wherein the alkyl (meth) acrylate comonomer is an alkylacrylate monomer with the alkyl moiety selected from C₁-C₄ alkyl groups.6. A cable as claimed in claim 5, wherein the alkyl moiety is methyl orbutyl.
 7. A cable as claimed in claim 4, wherein the trialkoxysilanetermonomer has the general formula III CH₂═CHSi(OA)₃  (III) wherein A isa hydrocarbyl group having 1-8 carbon atoms.
 8. A cable as claimed inclaim 4, wherein the trialkoxysilane termonomer comprises 0.2-5% byweight of the ethylene-alkyl (meth)acrylate-trialkoxysilane polymercomposition.
 9. A cable as claimed in claim 1, wherein the terpolymercomposition further includes from 0 up to 50% by weight of aplasticiser.
 10. A cable as claimed in claim 1, wherein the terpolymercomposition further includes up to 60% by weight of a filler.
 11. Acable as claimed in claim 1, wherein the terpolymer composition furthercomprises up to 10% by weight of an additive selected from the groupconsisiting of stabilisers, crosslinking agent, coagents, and processaids.
 12. A cable as claimed in claim 1, wherein said insulating andjacketing layers have a Shore A hardness of less than 85.